It is desirable to operate an air separation plant beyond the design capacity of the plant in order to produce extra products from the plant if such increased capacity operation can be carried out economically.
Most components of an air separation plant can be designed or modified to accept an increased flowrate. For example, an upstream blower can be used to boost the capacity of a compressor. Heat exchangers can be operated at increased flowrates simply by accepting an increased pressure drop. The capacity of air prepurifiers can also be increased by operating at increased pressure drop provided that fluidization of the adsorbent particles is avoided. However, it is more difficult to increase the capacity of the distillation columns in an air separation plant because they are capacity limited by the phenomenon of flooding. Flooding occurs in process equipment whenever there is vertical countercurrent two-phase flow and the flowrates are such that they exceed the capacity of the equipment. In both packed and trayed columns, the approach to flooding is characterized by a rapidly increasing pressure drop, by a loss of separation performance and by unstable operation. The onset of flooding in the columns is usually the limiting bottleneck encountered when attempting to increase the capacity of an air separation plant beyond its design capacity.
In general it is well established that distillation column capacity can be increased by changing the column pressure. Raising the pressure increases the vapor density, allowing an increase in the mass flowrate of vapor. However, increasing the pressure lowers the relative volatility thus making the distillation separation more difficult. The vapor mass flowrate capacity increases as the 0.4 or 0.5 power of the operating pressure for packed and trayed columns respectively.
The disadvantage of this solution to the flooding problem is that an increase in the column operating pressure translates into a substantial increase in the discharge pressure of the main air compressor, and in increased power costs. A pressure increase is particularly disadvantageous in the upper (or lower pressure) column of a double column plant since any increase in pressure must typically be multiplied by three as it is propagated across the main condenser/reboiler, because of the differences in the vapor pressure/temperature relationships of oxygen and nitrogen.
A solution to the problem is to increase the flowrates through the columns beyond the design point but not as far as the flood point. Typically packed columns are designed at about 80 percent of the flood point. Unfortunately, using conventional structured packing, flowrates can be increased only slightly beyond the design point because pressure drop fluctuations become so large that the columns become unstable.
Accordingly it is an object of this invention to provide a method for operating a cryogenic rectification column to carry out the separation of the components of air at increased capacity while avoiding flooding.